1. Field of the Invention
The present invention concerns-magnetic recording media suitable for high density recording, containing a carbon substrate, a magnetic recording layer, and at least one interlayer therebetween which provides one or more properties beneficial to preventing diffusion, inducing a preferred orientation in the magnetic recording layer and/or promoting adhesion between a carbon substrate and a barium hexaferrite-based magnetic recording layer. The present invention also concerns a process for producing the magnetic recording medium.
2. Discussion of the Background
Magnetic recording media such as-magnetic disks are now widely used for audio, video and computer applications. In a magnetic recording system, recording and reproducing are conducted by means of a magnetic head. A pattern of remanent magnetization is formed along the length of a track or a number of parallel tracks on a magnetic recording medium by means of the recording head. The recorded magnetization creates a pattern of magnetic fields which are used to read the data stored in the magnetic recording medium. When the recorded magnetic medium is passed by the same or a similar recording head, the pattern of magnetization can be read by the recording head and the recorded data reconstructed by appropriate electrical processing.
Recent efforts in the field of magnetic recording media have centered on the development of higher areal densities by increasing both the linear recording density as well as the track densities on the magnetic recording medium. Substantial increases in both linear and track densities require improvements in both magnetic recording materials and in recording techniques and components.
Magnetic recording requires an interaction between the surface of the magnetic recording medium and the recording head. High density recording requires that the distance between the magnetic recording medium and the recording head be as small as possible. With decreasing distances between the recording head and the surface of the recording medium, however, problems associated with the material properties of the magnetic head and the magnetic recording material arise.
For example, the surface of the magnetic recording medium must be finished to a mirror smooth surface to allow the recording head to approach the surface as closely as possible. At these close spacing distances, problems associated with friction and subsequent wear of a magnetic recording layer may result in deterioration of the recording head and medium. The surface smoothness and adhesion of magnetic recording layers to substrates on which they are deposited is, therefore, particularly important with high density recording media.
The standard substrate material for hard disk recording media is high-purity aluminum-magnesium alloy, coated with a nickel-phosphorus (Ni--P) alloy. Glass substrates have also been used, but suffer from problems associated with brittleness and fracture during assembly and operation (Magnetic Recording, C. D. Mee and E. D. Daniel, Vol. I, pp. 100 and 198-203, McGraw-Hill, Inc., 1987). More recently, carbon substrates have been proposed (U.S. Pat. Nos. 5,045,298 and 4,716,078).
Magnetic disks having high density memory capacity have been prepared by depositing a magnetic oxide or magnetic metal film onto the substrate surface. The medium film is conventionally deposited by means of a sputtering process (U.S. Pat. No. 4,411,963). Conventional sputtering processes include diode, triode and magnetron sputtering processes (Microchip Fabrication, P. Van Zant, pp. 204-208, Semiconductor Services, San Jose, Calif. 1984).
Magnetic recording materials may be utilized for both longitudinal (horizontal) and perpendicular (vertical) recording. Magnetic thin film thicknesses of 500-5,000 .ANG. are particularly preferred for perpendicular magnetic recording applications. Magnetic thin films having a thickness of about 200-800 .ANG. are particularly preferred for longitudinal recording applications. Many metal alloy thin films have been proposed for these applications (see, for example, U.S. Pat. Nos. 5,063,120, 5,084,152 and 4,654,276). Metal alloy films containing oxygen are also known (U.S. Pat. No. 5,066,552). These films may be deposited by conventional sputtering processes.
There are many compounds containing Ba, Fe and O. However, only one, barium ferrite (also known as barium hexaferrite, barium magnetoplumbite and/or BaFe.sub.12 O.sub.19) is useful for magnetic recording. Barium ferrite thin films have been investigated as possible magnetic recording layers for disk recording media. Ceramic barium ferrite has excellent hardness and resistance to environmental degradation, high magnetocrystalline anisotropy, high coercivity and a square hysteresis loop leading to high recording density. Barium ferrite films have also been produced on oxidized silicon wafers using a conventional rf diode sputtering system (A. Morisako, M. Matsumoto, and N. Naoe, IEEE Transactions on Magnetics, Vol. MAG-22(5):1146-1148, 1986). However, crystalline barium ferrite films generally require high substrate temperatures (400.degree.-650.degree. C.) during sputtering (Magnetic Recording, pp. 217).
The small distances between the recording head and the recording medium, critical to reaching higher densities on hard disks, give rise to disk wear, as noted above. Overcoat layers are known to provide a wear-resistant layer and to minimize disk wear. Overcoat layers prepared from rhodium, carbon, TiC, TiN, SiC, Cr.sub.2 C.sub.3 and Al.sub.2 O.sub.3 have been suggested (Magnetic Recording, pp. 219-222; and U.S. Pat. No. 4,789,598). A protective overcoat layer of hafnia and zirconia has also been proposed (U.S. Pat. No. 5,078,846).
Use of some metal nitrides (e.g., aluminum nitride or AlN) as interlayers in composite materials (such as an optical surface film) is known. For example, U.S. Pat. No. 5,270,263 discloses a process for depositing AlN using nitrogen plasma sputtering and the use of sputtered AlN as an etch stop in silicon semiconductor manufacturing. For this use, its etching properties with respect to other films are important. Uses of AlN as a passivation layer, a ceramic packaging material, a mask for ion implantation for Si and a window for GaAs solar cells are suggested. However, in U.S. Pat. No. 5,270,263, AlN is sputtered onto Si or GaAs, rather than carbon. A good lattice match to Si and GaAs is considered important.
U.S. Pat. No. 5,202,880 discloses AlN or Si.sub.3 N.sub.4 as a dielectric layer on a magneto-optical TbFeCo alloy medium, which permits recording on both sides of each substrate. The dielectric layer is placed between a reflective metal layer (Al) and the media layer to prevent corrosion and enhance the Kerr effect. Si.sub.3 N.sub.4 is used as an overcoat to protect the TbFeCo alloy medium from corrosion caused by the A1 reflective coating.
U.S. Pat. No. 5,132,238 discloses a thin film superconductor assembly, in which AlN is used as an electrically-insulating, thermally-conducting medium in a superconducting cable. AlN is applied between an aluminum substrate and a superconducting medium to electrically insulate one from the other while simultaneously providing good heat transport between the two. Aluminum oxynitride is also suggested as a possible material. Sputtering and direct nitridation of the aluminum-containing substrate are suggested methods for forming AlN.
U.S. Pat. No. 4,844,989 discloses a superconducting structure with layers of niobium nitride and aluminum nitride. The deposition of AlN/NbN multilayer films and their uses as a superconducting structure for power transmission, a Josephson junction and a microwave source are described. Methods of forming the multilayer films include sputtering. The selection of AlN and NbN was based on a good lattice match and the electrical insulating property of AlN.
U.S. Pat. No. 4,677,042 discloses a mask structure for lithography, in which AlN is used as a substrate. The mask is used in X-ray lithography of very fine structures. The AlN properties of interest are the high X-ray and visible light transmittance, low coefficient of thermal expansion, high thermal conductivity and ease of film formation. The films can be multilayers of polymers and AlN or BN/AlN, and the thickness of the film is 20 microns.
However, it is believed that the use of AlN as an intermediate film in a magnetic recording medium was not known prior to the present invention.
Thin films of a different metal nitride, TiN, have been used as a diffusion barrier between metals and silicon (see U.S. Pat. Nos. 5,279,857, 5,279.,985, 5,277,985, 5,275,715, 5,268,590, 5,250,467, 5,242,860 and 5,240,880).
Oxide films have also been used as interlayers in composite films. For example, Al.sub.2 O.sub.3, an extensively studied material, has been deposited by sputtering or CVD as a thin film (see Ohring, "The Materials Science of Thin Films," Milton Academic Press, Inc., Boston, 1992, page 547ff).
An orientation-inducing interlayer is described in a magnetic recording medium containing a barium ferrite film. For example, oriented barium ferrite thin films can be grown on SiO.sub.2 /Si wafer disks on which a c-axis-oriented ZnO film has been deposited (M. Matsuoka, M. Naoe, and Y. Hoshi, J. Appl. Phys., 57(1):4040-4042, 1985). Oriented barium ferrite thin films are obtained using a facing target-type sputtering system and a substrate temperature of 500.degree. C. However, in the case where zinc oxide was deposited on a silicon wafer, no cleaning of the surface was reported. Accordingly, a thin silicon oxide surface layer may have been present in the silicon wafer substrate containing onto which a ZnO layer was deposited.
Substitution of certain elements or ions for iron atoms or ions in barium ferrite to modify its magnetic properties is known (see "Ferromagnetic Materials," vol. 3, E. P. Wohlfarth, ed. North Holland, Amsterdam (1982), referring to Kojima, "Fundamental Properties of Hexagonal Ferrites with Magnetoplumbite Structure," pp. 367ff). In general, the requirements for a replacement atom or ion in barium hexaferrite are that it fit-into the iron site of the magnetoplumbite lattice without significant distortion, and that the average formal charge be 3+ to retain electroneutrality.
A need exists for improved magnetic recording media (and in particular, disk magnetic recording media) which are useful for high density magnetic recording, and in which one or more interlayers provide one or more properties beneficial to preventing diffusion, inducing a preferred orientation in the magnetic recording layer and/or promoting adhesion between a carbon substrate and a barium hexaferrite-based magnetic recording layer.